Radiopharmacy and Radiopharmaceutical Chemistry
Generator chemistry and quality control
A Tc-99m generator is an alumina column loaded with Mo-99; saline elutes pertechnetate, and because Mo-99 (66 h) is the parent, the system reaches transient equilibrium in about 24 h, when Mo activity slightly exceeds Tc activity at elution. Aluminium breakthrough from the column is measured colorimetrically and degrades labeling; molybdenum breakthrough is assayed separately and is limited to 0.15 kBq Mo-99 per MBq Tc-99m. Sr-82/Rb-82 and Ge-68/Ga-68 generators carry analogous purity limits.
Distinguish three purities. Radionuclidic purity is the fraction of activity present as the intended radionuclide. Radiochemical purity (RCP) is the fraction of that activity in the intended chemical form. Chemical purity is the absence of nonradioactive contaminants. RCP is measured by ITLC, paper chromatography, or HPLC, and each solvent system separates a different impurity, so one strip rarely suffices.
Technetium chemistry depends on stannous-ion reduction of pertechnetate; kits are lyophilized and have short post-reconstitution expiry. Oxidized stannous ion, introduced air, or excessive eluate activity lowers labeling efficiency. Know the workhorses: MDP for bone, MAG3 (tubular secretion) and DTPA (GFR) for kidney, sulfur colloid for liver/spleen and marrow, sestamibi/tetrofosmin for myocardium and parathyroid, HMPAO/ECD for brain, and RBCs for GI bleeding. Half-lives to memorize: F-18 110 min, Ga-68 68 min, Tc-99m 6 h, I-123 13.2 h, In-111 67 h, I-131 8.0 d, Lu-177 6.7 d, Ra-223 11.4 d.
Traps: conflating radionuclidic with radiochemical purity; forgetting aluminium (not molybdenum) is the column contaminant that ruins labeling; using a kit past expiry; assuming two agents of the same nuclide are interchangeable.
Worked example: an ITLC strip counts 100,000 cpm total with 94,000 cpm at the origin and pertechnetate as the only mobile species, giving RCP = 94% - above a typical 90% release threshold.